1. Field
The disclosed embodiments relate to the design of DC/DC converters. More specifically, the disclosed embodiments relate to designing bootstrap capacitor refresh circuits for synchronous switching DC/DC converters.
2. Related Art
Switched-mode power converters (or “switching converters”) are a type of DC/DC power converter which incorporates a switching regulator to convert electrical power from one DC voltage to another DC voltage more efficiently. Switching converters are commonly used in modern computing devices (e.g., both desktop and laptop computers, tablet computers, portable media players, smartphones, and/or other modern computing devices), battery chargers, and electrical vehicles, among other applications. Synchronous switching converters are a particular type of switching converter which utilizes both a high-side MOSFET and a low-side MOSFET to perform a synchronous switching operation.
Switching converters can be classified according to the circuit topology. A buck-type switching converter is a step-down switching converter with an output voltage less than the input voltage level. A boost-type switching converter is a step-up switching converter with an output voltage greater than the input voltage level. A buck-boost switching converter is a DC/DC converter that has an output voltage that can be either greater than or less than the input voltage level. Note that a buck-boost switching converter can be formed by a buck switching converter followed by a boost switching converter. Hence, a buck synchronous switching converter uses both a high-side MOSFET and a low-side MOSFET to perform a synchronous step-down switching operation; a boost synchronous switching converter uses both a high-side MOSFET and a low-side MOSFET to perform a synchronous step-up switching operation; and a 4-switch buck-boost synchronous switching converter can be formed by the two high-side MOSFETs and the two low-side MOSFETs from both the buck and the boost converters.
In practice, each of the buck, boost and buck-boost synchronous switching converters can be controlled by pulse-width modulation (PWM) signals to further improve converter efficiency and to achieve desired output voltage levels. When a synchronous switching converter performs a PWM switching operation, a bootstrap capacitor (CBOOT) is often used to provide energy to turn on/off the high-side MOSFET. As the bootstrap capacitor discharges and voltages across the capacitor drop, the bootstrap capacitor has to be refreshed to maintain a sufficient operational voltage. The operation to keep the voltage across the bootstrap capacitor at certain range is referred to as “refresh,” and is traditionally achieved by coupling a supply voltage Vs to CBOOT through a diode. More specifically, the energy is delivered from VS to CBOOT when low-side MOSFET turns on. Limited by total impedance of the refresh loop, this refresh operation demands enough turn-on time of the low-side MOSFET. However, these conventional refresh techniques for CBOOT refresh have a number of drawbacks.
Using the 2-switch buck converter as an example, note that during a discontinuous-current-mode (DCM) PWM switching operation, the turn-on time of the low-side MOSFET is determined by the load conditions. More specifically, when the load is high, the average inductor current IL is also high and the turn-on time of the low-side MOSFET is longer. However, when the load is light, inductor current IL drops to near zero, thus the turn-on time is very short, and even zero (some inductor current detection techniques turn off the low-side MOSFET when IL is near zero). Consequently, there is not enough time to charge up CBOOT to a sufficiently high voltage in light load conditions using the conventional refresh techniques. As mentioned above, to have a successful refresh, the turn-on time of low-side MOSFET needs to be sufficiently long, which means the refresh pulse of low-side MOSFET can be longer than the pulse needed for the DCM operation. This requirement can force the inductor current IL into negative territory at some light load conditions. However, such a negative current is not desirable for the DCM operation. For example, when the low-side MOSFET is turned off after the refresh cycle, the negative inductor current can flow into the input side of the buck converter through the high-side MOSFET. Effectively, this condition transfers energy from the output back to the input and causes the input voltage to increase (referred to as a “boost-back” condition in a buck converter). This undesired input voltage increase can lead to over-voltage stress. Additionally, when the converter output has an energy-storage component such as a battery, the boost-back condition can also cause unwanted battery discharges current. Similar problem happens when the converter is transitioned into performing pulse-frequency modulation (PFM) switching at a light load condition, wherein the switching frequency is significantly reduced (e.g., down from a few hundred kHz to a few kHz), and the refresh pulse, as well as the negative induct current can happens in the time interval between two PFM pulses.
Another drawback of the conventional refresh techniques is associated with a 4-switch buck-boost converter. In particular, there exists an operation mode where the 4-switch buck-boost converter is under boost-mode operation, wherein the two boost MOSFETs Q3/Q4 are under PWM switching while the two buck MOSFETs Q1/Q2 are not under PWM switching. Instead, the high-side MOSFET Q1 needs to be always ON, while the low-side MOSFET Q2 needs to be always OFF. However, to refresh the bootstrap capacitor requires the high-side MOSFET Q1 to be turned off while the low-side MOSFET Q2 is turned on. Unfortunately, in the case of a 4-switch buck-boost converter, allowing such a refresh operation to occur means that normal operation has to be interrupted. Note that the same problem also occurs when the buck-boost converter is under buck-mode operation.
Hence, what is needed is a synchronous switching DC/DC converter design without the above-described problems.